Hmmm...it's tricky enough to try to find footage of the first king of Old Trafford, Denis Law, on Youtube. So, what more with Billy Garton? If I'm not mistaken, he played for United right before Fergie made United a powerhouse again (1980's).
I'd try to hit the Man United fan forums like Big Soccer's or a forum where you're likely to find a hardcore United fan base: Red11.org or Republikofmancunia.org. I'm sure a few have some relatively obscure United clips.
Hope that helped.